Google Apps for email (Gmail for your domain) blocks some legitimate mail by default
If you are using Google Apps for your domain name and gmail is your email service, then you aren’t getting important messages sent to “abuse” and “postmaster”.
Google intercepts all abuse@domain and postmaster@domain email and does NOT provide you with a copy by default! Thanks Google, ugh.
This means if your website is accidentally spamming hotmail for example, Microsoft will simply block your website from sending to all hotmail users, because you never got their warnings about your server sending spam.
However you can fix this behavior:
Basically you have to create a group for “abuse” and a group for “postmaster” and then make your email address the owner of that group.